Extreme Sea Levels Predicted to Increase

A new study has predicted that future global warming will lead to an increase in extreme sea levels, with consequent flood risks to coastal infrastructure and human populations.

Extreme sea levels occur through a combination of high tides and extreme weather events, which can generate storm surges and high wind waves. These phenomena are exacerbated by progressive rises in mean sea level and predicted increases in tropical cyclone activity.

Researchers have taken account of all these processes for the first time to assess future risk of extreme sea levels out to the year 2100. They predict that the extreme sea level events that occur, on average, once every 100 years at the present day, would occur almost annually along most global coastlines by the end of the century.

The research team also calculated that the impacts of extreme sea level were significantly reduced in scenarios where CO2 emissions were mitigated compared to a business-as-usual scenario, largely due to reduced thermal expansion of the oceans and lower rates of glacier and ice sheet melting.

The research was undertaken by the National Oceanography Centre and an international research team, from Italy, Greece, the Netherlands and the U.K.

Dr. Svetlana Jevrejeva from the National Oceanographic Centre, said: “The projected rise in extreme sea levels will increase the frequency of catastrophic flooding events along many of the world’s coastlines, with massive socio-economic impacts. The global coastline covers some 620,000 kilometers (385,000 miles), so mitigating the impacts of increased extreme sea levels will be a major challenge.”

The new results indicate that upgrades to existing coastal defenses would require an average increased height of at least 50 centimeters (20 inches) by 2100, although locally this may need to be in the order of one to two meters. Without sufficient mitigation the impacts on human populations could be significant, especially as by 2100 it is estimated that up to 500 million people will be living in coastal areas less than five meters above sea level.

A further study led by the National Oceanographic Centre found flooding from rising sea levels could cost $14 trillion worldwide annually by 2100, if the target of holding global temperatures below 2ºC above pre-industrial levels is missed.

The researchers also found that upper-middle income countries such as China would see the largest increase in flood costs, whereas the highest income countries would suffer the least, as a result of existing high levels of protection infrastructure.